The invention relates to a pressure sensor, a pressure measuring apparatus which comprises such a pressure sensor, and a method for monitoring the pressure in a chamber by means of such a pressure sensor or pressure measuring apparatus.
Known pressure sensors of the generic type (cf. for example EP-A-0 351 701) contain only a capacitive measuring element consisting of a support plate and a membrane, which carry electrically conductive layers. Although measurement can be carried out independently of the type of gas and with high accuracy between about 10xe2x88x926 mbar and 10 bar by means of such measuring elements, the total range cannot be measured with a single measuring element. Furthermore, such pressure sensors always have an offset problem which can cause considerable inaccuracies, particularly in the longer term.
The use of pressure sensors which are in the form of a thermal conductivity measuring element, for example according to Pirani, is also known. In such pressure sensors, at least one heating element, usually a measuring wire, is electrically heated and the pressure is determined from the heating power, making use of the pressure-dependent thermal conductivity of the gas. In this way, it is possible to measure the pressure in a range between about 10xe2x88x923 mbar and a few 100 mbar. Above a few 10 mbar, however, convective heat transmission predominates, so that the measurement is influenced there by gas flows and is highly position-dependent. Moreover, measurement by this method is always dependent on the type of gas. A heat-conduction sensor, including its evaluation circuit, can also be designed in such a way that it can be operated up to about 10xe2x88x925 or max. 10xe2x88x926 mbar, but in this case higher pressures above a few 10 mbar can no longer be reliably measured.
It is also known that so-called ionization sensors whose function is based on the measurement of the particle current density can be used for pressures below 10xe2x88x922 mbar and, with further reduced accuracy, up to 10xe2x88x921 mbar. A distinction is made between cold-cathode ionization vacuum gauges and those having a hot cathode. They are not capable of functioning in higher pressure ranges and are inaccurate from about 10xe2x88x922 mbar. They are in principle dependent on the type of gas.
If it is intended to measure large pressure ranges, for example from about 10xe2x88x926 mbar to about 100 mbar, it is usual to use at least two different, spatially separated pressure sensors which, independently of one another, are each also provided with devices for processing the measuring signal. Thus, for example, two or more pressure sensors which each contain a capacitive measuring element of the type described at the outset which is suitable for measuring a part of the range can be used. These and similar solutions are, however, expensive owing to the associated technical complexity. The distance between the measuring elements can lead to uncertainty in the measured result. Moreover, the offset problem persists even when a plurality of such measuring elements are used.
However, the combination of different pressure sensors in one apparatus is also known. Thus, EP-A-0 658 755 discloses a pressure measuring apparatus in which a Pirani heat-conduction measuring element and a cold-cathode measuring element are combined to give a pressure measuring apparatus, the former measuring an upper pressure range and the latter a lower one. Although the apparatus is compact and can measure the total above-mentioned measuring range, it is very inaccurate in the upper part.
The combination of a bellows-type mechanical measuring element with a Pirani-like thermal conductivity measuring element is also known (U.S. Pat. No. 3,064,478). Here, the corresponding pressure measuring apparatus is relatively inconvenient. Moreover, the spatial separation of the measuring elements may give rise to uncertainties in the measured result.
It is the object of the invention to provide a pressure sensor which covers a large measuring rangexe2x80x94preferably from about 10xe2x88x926 mbar to a few barxe2x80x94and at the same time has a simple design and is compact and economical and insensitive to contamination. This object is achieved by the features in the claims.
It is also intended to provide a pressure measuring apparatus which is distinguished by the same characteristics as the pressure sensor according to the invention and which can be mounted easily, in any position and quickly, and furthermore a pressure measuring apparatus which combines these advantages with a very large measuring range with high accuracy and stability.
Finally, it is intended to provide a method for monitoring the pressure in a chamber, which is sufficiently accurate and stable in the long term over a large measuring range.
The pressure sensor according to the invention combines a capacitive measuring element with a thermal conductivity measuring element in compact, easily handled and economically producible form. The upper pressure range above about 0.1 mbar is covered by the capacitive measuring element independent of the type of gas, and the lower pressure range from about 10xe2x88x926 mbar to about 10 mbar by the thermal conductivity measuring element, for example averaging being performed in the overlap region. Consequently, pressures between about 1 mbar and a few bar can be measured independently of the type of gas and with high accuracy (in general about 1%), while at the same time the measuring range extends down to about 10xe2x88x926 mbar with generally sufficient accuracy of measurement.
The proximity of the two measuring elements ensures that they are always exposed to the same conditions. The pressure sensor is versatile and may also be formed in such a way that it is suitable for relative pressure measurements in the first-mentioned part of the measuring range. The arrangement can be designed to be small and compact, for example having a diameter of 35 mm or less. Regarding the thermal conductivity measurement, optimal operating behaviour can be realized in said pressure range with a heating element of small size, for example a short heating filament.
The pressure measuring apparatuses according to the invention have the advantages of the pressure sensor according to the invention and are convenient and easy to mount. When supplemented by suitable further measuring elements, they have a compact design and can measure the pressure in a wide measuring range with high accuracy.
The method according to the invention makes it possible to monitor the pressure in a chamber in a stable manner over a long time, in spite of the offset problems of capacitive measuring cells. These advantageous characteristics are displayed in particular in the monitoring of the pressure in locks since, owing to the cyclic pressure changes, the offset can be regularly compensated here.